Switchable optical elements including shutters, diaphragms etc. based on mechanical moving parts are known. Switchable optical elements based on mechanical moving parts have optical transmission properties that can be switched between predetermined states of differing optical transmittance, for example, states of high or low amplitude attenuation. However, mechanical moving parts can be relatively large and expensive to produce. Furthermore, mechanical moving parts are subject to wear, so have a limited lifespan and may suffer from reliability problems during their lifespan.
The U.S. patent application 2001/0017985 describes an optical device which does not require mechanical moving parts and has optical transmission properties that can be switched between predetermined states. FIGS. 12 and 13 show the device 100, which comprises a substantially opaque first fluid 101 and a substantially transparent second fluid 102 sealed in a container 103. The fluids do not mix, and are arranged in the container 103 such that their boundary 104 forms a section of a substantially spherical surface. Using electro capillarity (electro-wetting) the shape of the boundary 104 between the first and second fluid 101,102 can be changed between the arrangement of FIG. 12, and that of FIG. 13.
In the arrangement of FIG. 12 no voltage is applied to the device 100, and light incident on the device 100 is blocked by the substantially opaque first fluid 101. Applying a voltage to the device gives rise to the arrangement of FIG. 13. In FIG. 13 the boundary 104 between the first and second fluids 101,102 is shaped so that the first fluid 101 is be excluded from the optical path of the device 100. The shape of the boundary 104 is controlled so that the boundary 104 contacts an interior surface of the container 103, forcing the first fluid 101 away from the area of contact. Light incident on the device 100 is therefore not blocked by the first fluid 101, and the device 100 is substantially transparent.
The device 100 suffers a number of drawbacks. Firstly, the device 100 is relatively inflexible as the shape of the area of contact is hard to control to be anything other than circular. Secondly, in practice it may be difficult to separate the second fluid 102 from the interior surface of the container 103 when changing from an on state (FIG. 13) to an off state (FIG. 12). The separation and adhesion of the second fluid 102 on the interior surface will, most likely, show a non-monotonic transition.
Thirdly, in the off state the curvature of the interface 104 between the two fluids means that the device may not have uniform transmission characteristics across its area. Fourthly, due to the curved surface, the optically active area is significantly smaller than the actual size of the device. Fifthly, as the boundary 104 between the two fluids is generally curved, the fluids must be of substantially equal refractive index to avoid introducing a lens effect at the boundary. Other demands are placed on the properties of the fluids to enable the electro-wetting effect to take place, and to give the desired variability in transmittance characteristics. Therefore, the range of fluids available for use in the device is relatively restricted, and to achieve a working device compromises may have to be made on some properties.
It is an aim of preferred embodiments of the present invention to provide a switchable optical element, which addresses one or more of the problems of the prior art, whether referred to herein or otherwise. A further aim is to provide a method of manufacturing such an element.